Synchronous Rectifier

ABSTRACT

The disclosed embodiments relate to apparatus and method for reducing power losses in a power supply. There is provided an apparatus comprising means ( 214 ) for coupling a first signal (S 2 ) to a reference level (ground) when the coupling means is conductive, means ( 202, 204 ) for placing the coupling means in a conductive state during a duration of a portion of a period of a second signal (S 3 ), and means ( 206 ) for altering the duration of conduction of the coupling means in response to an amplitude of the second signal.

FIELD OF THE INVENTION

This invention relates generally to the field of power supplies and more specifically to the use of synchronous rectification in switch-mode power supplies.

BACKGROUND OF THE INVENTION

As shown in FIG. 1, a typical switch-mode power supply (SMPS) includes primary side components 150 and secondary side components. Primary side, also referred to as “hot side” components comprise switch-mode controller 106, a switch-mode metal-oxide-semiconductor (MOSFET) 108, MOSFET heatsink 110, current sensing resistor 112, surge suppression capacitor 114, a transformer 116 having primary winding 120, secondary winding 118, rectifier diode 102, filter capacitor 104 and an opto-isolator 126. Secondary or “cold-side” components comprise secondary transformer windings 122 and 124, rectifier diodes 128 and 136 with their respective heatsinks 130 and 138 and filter capacitors 132 and 134. The entire switch-mode supply is powered from an unregulated voltage source 100. The controller 106 provides a drive signal V_(D) to MOSFET 108 to produce current flow in primary winding 120 of transformer 116. Secondary winding 118 of transformer 116 provides a source of voltage, which when rectified and filtered by diode 102 and capacitor 104 respectively, provide supply voltage V_(DD) to controller 106. Feedback signal V_(FB) is developed from a rectified and filtered secondary supply +12V and fed back to controller 106 through opto-isolator 126, thus establishing a feedback loop to control the switching on and off of MOSFET 108. By comparison of feedback signal V_(FB) to a reference value in controller 106 and variation of the conduction cycle of MOSFET 108 in response to differences between the feedback signal and the reference level, regulation of operating levels in the SMPS can be realized. Resistor 112 senses the primary current flowing in MOSFET 108 which serves as the current feedback signal to current-mode controller 106. Using current-mode control prevents excessive current to be drawn from the switch-mode supply under overload conditions. By rectifying a signal from transformer 116 secondary windings 122 and 124 by diodes 128 and 136 respectively, regulated output voltages +6.5V and +12V are developed and filtered by capacitors 132 and 134 respectively. Rectification of the signals developed across windings 122 and 124 may be accomplished by diodes in series with their respective windings between ground and the supply outputs. In this described typical SMPS, one of the diodes, 128, is placed with its cathode connected to the positive output of its particular supply, thus causing both anode and cathode of diode 128 to be remote from ground. In the exemplary +12V supply, diode 136 is placed such that its anode is connected to ground. In the type rectifiers described in this exemplary switch-mode supply, an often large source of inefficiency is the voltage drop across the rectifier diodes. In higher power supplies the inefficiency introduced by the voltage drop across the diodes can be significant, thus requiring heat sinking and possibly active measures such as forced air cooling.

In order to improve the rectifier efficiency, a transistor, usually a MOSFET may be used as a low voltage-drop switch to replace a diode. This technique is referred to as synchronous rectification. Synchronous rectification requires control of the drive to the synchronous rectifier to turn the MOSFET on or off during the appropriate portions of the signal being rectified. Integrated circuit controllers are often used to control conduction of the MOSFET. These integrated circuits, such as the ST Microelectronics STS-R3, or Anachip AP436 are moderately expensive and require an additional 4 to 8 external components. These ICs often include clock generation circuits and other sophisticated methods to determine on/off control of synchronous rectifier MOSFETS. The present invention involves a less complex control circuit that may use discrete components to provide a low-cost implementation of synchronous rectification in a switch-mode power supply.

SUMMARY OF THE INVENTION

Certain aspects commensurate in scope with the originally claimed invention are set forth below; however the invention may encompass a variety of aspects that may not be set forth below.

The disclosed embodiments relate to an apparatus comprising a first device, possibly a transistor, configured to couple a first signal to a reference level when the first device is made conductive, a second device, possibly a differentiator or a high-pass filter, responsive to a second signal which may be out of phase with the first signal, the second device configured to control conduction of the first device during a portion of a period of the second signal, and a detector, possibly a diode peak detector, responsive to an amplitude of the second signal, the detector configured to alter a duration of conduction of the first device. In this apparatus the detector may reduce the duration of conduction of the first device in response to an increase in the amplitude of the second signal.

A further embodiment includes means for coupling a first signal to a reference level when said coupling means is conductive, means for placing said coupling means in a conductive state during a duration of a portion of a period of a second signal, and means for altering said duration of conduction of said coupling means in response to an amplitude of said second signal.

A yet further embodiment is a method comprising the steps of coupling a first signal to a reference level by conduction of a device, differentiating a second signal, controlling conduction of the device in response to the differentiated second signal, and altering conduction of the device in response to an amplitude of the second signal. Variations of this method may include reducing the duration of conduction of the device in response to an increase in the amplitude of the second signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings in which similar elements in each figure have the same reference designator:

FIG. 1 is a block diagram of a typical switch-mode power supply;

FIG. 2 is a schematic diagram of an embodiment of the present invention;

FIG. 3 is a representative waveform of the drain voltage of the main switching MOSFET (108); and

FIG. 4 shows representative waveforms of the drain voltage and gate voltage of a synchronous rectifier MOSFET in an embodiment of the invention.

DETAILED DESCRIPTION

An embodiment of the discrete control circuit shown in FIG. 2 satisfies the need for a low-cost, synchronous rectifier controller. FIG. 2 shows a representative switch-mode power supply used in electronic equipment applications. The primary side circuit 150 is typical of switch-mode power supplies, is well known to those skilled in the art and is similar to that described previously. Switch-mode transformer 116 has multiple secondary windings, 122 and 124, to develop different supply voltages. Diode 128 is used in this system as a conventional rectifier. Use of this high-side rectifier for the +6.5V supply serves the dual purposes of rectifying the signal S₃ from winding 122 to generate the +6.5V supply and so that the AC signal, S₃, at the anode of diode 128 will be available to develop a switching control signal to drive synchronous rectifier MOSFET 214. MOSFET transistors can be used as rectifiers (synchronous rectification) by controlling the conduction time of the MOSFET, making the conduction coincide with the desired portion of the pulse waveform. Since the MOSFET can have much lower voltage drop than even a Schottky diode, the efficiency of the power supply can be improved. In most cases, when synchronous rectification is used, it is possible to eliminate the large heatsink normally used to cool the diodes. In the exemplary embodiment shown in FIG. 2, MOSFET 214 is arranged with its source connected to the secondary side ground. This configuration simplifies generating the drive signal to MOSFET 214. The control voltage for the gate drive of synchronous rectifier 214 is developed from the signal S₃. The pulse signal S₃ is of the opposite polarity to signal S₂ that appears at the drain of MOSFET 214. This phase reversal is determined by the phasing of windings 122 and 124 and is done so that the polarity of signal S₃ is of the phase needed to turn on the MOSFET gate when signal S₂ is at its most negative level. Conduction from the drain to source of MOSFET 214 when signal S₂ is at its most negative level clamps signal S₂ to ground, thus rectifying signal S₂ to produce the +12V output. Controller 106 is designed such that signals S₂ and S₃ can have variable duty cycles; with the positive portion of signal S₃ increasing in duration at higher line voltage. As a result, means are necessary for shortening the duration of the pulse at MOSFET 214 gate so as to provide the proper conduction time for the MOSFET to assure that MOSFET 214 conducts only when signal S₂ is at its negative level. Capacitor 202 and resistor 204 form a high-pass filter that differentiates the waveform on signal S₃ to produce the drive waveform for the MOSFET 214 gate. Differentiation of the waveform aids to reduce the conduction time of MOSFET 214 so that MOSFET 214 turns on at or after the voltage at its drain is negative and turns off at or before its drain voltage goes high. A low power (by comparison to a conventional rectifier diode such as diode 128) diode 136 conducts during the negative excursion of signal S₂ during which time the MOSFET may not be turned on due to the gate drive interval to MOSFET 214 possibly being of lesser duration than the negative excursion of signal S₂. The function just described for diode 136 could also be performed by an internal parasitic diode in MOSFET 214. Diode 206 and capacitor 208 rectify signal S₃ to develop a negative bias voltage that is proportional to the value of unregulated voltage source 100 (and to the AC line input voltage). As the negative bias increases it reduces the average voltage at the gate of MOSFET 214 and thus reduces the conduction time of the MOSFET. The voltage divider formed by resistors 210 and 204 scale the negative bias developed by diode 206 and capacitor 208 to establish the desired range of negative bias added to the gate drive. This negative bias on the gate of MOSFET 214 prevents excess conduction (and increased losses) at high line voltages. Resistor 200 provides current limiting for the negative supply formed by diode 206. Resistor 212 reduces the rise time of the drive voltage to MOSFET 214 gate to minimize radiated noise due to fast switching transients.

The waveform depicted in FIG. 3 shows the signal S₁ at the drain of switch-mode MOSFET 110, this voltage being the primary voltage on winding 120 of transformer 116. The upper trace of FIG. 4 depicts the signal voltage S₂ which is induced in secondary winding 124, this being also the voltage on the drain of synchronous rectifier MOSFET 214. The lower trace of FIG. 4 depicts the gate drive to MOSFET 214. The conduction threshold of MOSFET 214 is approximately 2.5 volts to 3.0 volts, as shown by R₁ and R₂ in FIG. 4. Projecting the point in time of the gate drive signal passing the conduction threshold onto the drain voltage waveform in the upper plot of FIG. 4 indicates that the MOSFET 214 conduction is well within the interval of the negative excursion of signal S₂. It can also be seen from the gate drive waveform in FIG. 4 that increasing the negative bias applied to the gate of MOSFET 214 through detector 206, as occurs at higher line voltage, will reduce the period of time that the gate drive voltage is above the conduction threshold R₁ or R₂.

While the present invention has been described with reference to the preferred embodiments, it is apparent that various changes may be made in the embodiments without departing from the spirit and the scope of the invention. 

1. An apparatus, comprising: a first device (214) configured to couple a first signal (S₂) to a reference level (ground) when said first device (214) is made conductive; a second device (202,204) responsive to a second signal (S₃), said second device (202,204) configured to control conduction of said first device during a portion of a period of said second signal; and a detector (206) responsive to an amplitude of said second signal (S₃), said detector configured to alter a duration of conduction of said first device (214).
 2. The apparatus of claim 1 wherein said detector reduces said duration of conduction of said first device in response to an increase in said amplitude of said second signal.
 3. The apparatus of claim 1 wherein said first device (214) is a transistor.
 4. The apparatus of claim 3 wherein said transistor (214) is a metal-oxide-semiconductor.
 5. The apparatus of claim 1 wherein said second device (202,204) is a differentiating device.
 6. The apparatus of claim 5 wherein said differentiating device comprises a high-pass filter (202,204).
 7. The apparatus of claim 6 wherein said high-pass filter comprises a capacitor and a resistor (202,204).
 8. The apparatus of claim 1 wherein said first signal (S₂) is of a different phase from said second signal (S₃).
 9. The apparatus of claim 8 wherein said first signal (S₂) is of an opposite phase from said second signal (S₃).
 10. The apparatus of claim 1 wherein said detector (206,208) is responsive to a negative peak value of said second signal (S₃).
 11. The apparatus of claim 10 wherein said detector comprises a diode.
 12. A method, comprising the steps of: coupling a first signal (S₂) to a reference level (ground) by conduction of a device (214); differentiating a second signal (S₃); controlling conduction of said device (214) in response to said differentiated second signal; and altering conduction of said device (214) in response to an amplitude of said second signal (S₃).
 13. The method of claim 12 wherein said duration of conduction is reduced in response to an increase in said amplitude of said second signal.
 14. The method of claim 12 wherein said device comprises a transistor.
 15. An apparatus, comprising: a first source (124) of a first signal (S₂) and a second source (122) of a second signal (S₃), said first signal (S₂) and said second signal (S₃) being of different phases; a transistor (214) having a first main current conducting terminal (drain) and a second main current conduction terminal (source) and a conduction control terminal (gate), said first main current conduction terminal connected to said first signal (S₂) and said second main conduction terminal connected to a reference level (ground); a capacitor (202), a first terminal of which is connected to said second signal and a second terminal of which is connected to said conduction control terminal; a resistor (204), a first terminal of which is connected to said conduction control terminal and a second terminal of which is connected to said reference level; and a diode (206), a cathode of which is connected to said second signal (S₃) and an anode of which is connected to said conduction control terminal.
 16. The apparatus of claim 15 wherein said first source (124) and said second source (122) are windings of a switch-mode transformer (116).
 17. The apparatus of claim 15 wherein said transistor is a metal-oxide-semiconductor.
 18. An apparatus comprising: means (214) for coupling a first signal (S₂) to a reference level (ground) when said coupling means is conductive; means (202,204) for placing said coupling means in a conductive state during a duration of a portion of a period of a second signal (S₃); and means (206) for altering said duration of conduction of said coupling means in response to an amplitude of said second signal.
 19. The apparatus of claim 18 wherein said coupling means (214) comprises a transistor.
 20. The apparatus of claim 18 wherein said placing means (202,204) is a differentiator.
 21. The apparatus of claim 20 wherein said differentiator comprises a high-pass filter.
 22. The apparatus of claim 18 wherein said altering means reduces said duration of conduction of said coupling means in response to an increase in said amplitude of said second signal. 